Pressure Relief Valve (PRV) | SKYbrary Aviation Safety

Description

Pressure relief valves are essential components in an aircraft's fuel, hydraulic, and pneumatic systems. These valves perform three critical functions: safety relief, pressure regulation, and protection against thermal expansion.

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Safety Relief Valves

Safety relief valves are fast-acting components designed to protect systems from sudden pressure spikes. These valves may remain inactive for extended periods, making it vital for them to function correctly when needed.

Pressure Regulating Relief Valves

In contrast to safety relief valves, pressure regulating valves operate more frequently. They counterbalance momentum forces that disrupt the normal force equilibrium of typical direct-acting valves.

Thermal Relief Valves

When transitioning from the cold environment at 30,000 feet to a warm desert runway, rapid temperature changes can lead to the thermal expansion of fluids, potentially damaging the system. Thermal relief valves are designed to release small volumes of fluid to mitigate the risks associated with thermal expansion.

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Positive and Negative Pressure Relief Valves

Aircraft are engineered to endure higher internal pressure compared to external ambient air pressure without damage. However, there is a maximum pressure limit that they can tolerate. A positive pressure relief valve opens when the internal pressure exceeds desired levels to prevent damage. It typically activates if the cabin pressure becomes too high relative to the outside air pressure (a pressure differential greater than 8.85 psi / 0.61 bar), often due to a malfunction of the outflow valve, which remains closed.

Conversely, aircraft are not designed to handle external pressures that exceed internal pressures as robustly. The negative pressure relief valve prevents the creation of sub-atmospheric pressures in the system, which can result from an imbalance between fresh gas intake and gas evacuation rates. This scenario, while rare, can occur during rapid descents when the cabin altitude remains at 8,000 feet (normal conditions), while the aircraft itself descends below that altitude.

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